–But active conditions to be expected.–

Although the Atlantic hurricane season should currently be at its peak, it has been unexpect­edly quiet. However, there are many conditions that in­dicate that the latter part of the hurricane season is going to be active, states the Royal Netherlands Meteorological Institute KNMI in an article published on Tuesday, Sep­tember 10.

This is because the seawater is still very warm; large parts of the Atlantic Ocean have seawater temperatures (well) above 27 degrees Celsius. A hurricane that has managed to form and can come in contact with that warm wa­ter has much energy at its disposal, meteorologists ex­plain. In addition, the tropi­cal region is experiencing a La Nina, the counterpart of El Nino, whereby there tends to be more hurricane activ­ity. This La Nina is expected to continue toward the final months of the season, KNMI writes.

In recent weeks, there were very few tropical waves over the Atlantic, with the result that no hurricanes formed either, writes KNMI. It did rain in the Sahara, however. Tropical rain clusters usu­ally move toward the Atlan­tic Ocean from the African mainland, but they now sud­denly moved toward the Sa­hara. As a result, dry desert areas were surprised with a lot of rain. Last weekend, for example, there were floods in Morocco and above-average precipitation also fell in the usually dry Mali and Mauri­tania.

This hurricane season, the Atlantic region experienced its earliest category 4 and 5 hurricane ever: hurricane Beryl formed in late June and quickly proved to be one of historic proportions, form­ing in an area where nor­mally such severe hurricanes could not yet form this early in the season.

After Beryl fizzled out, the Atlantic Ocean became calm, explains KNMI. This was partly because there was a lot of Sahara sand hang­ing over the Atlantic Ocean, preventing hurricanes from forming easily. This is be­cause the dry air from the Sahara causes the hurricane to lose its supply of warm, moist air. In addition, the Sa­hara sand causes sunlight to be reflected and the seawater to become slightly cooler, re­ducing the energy available to the hurricane.

Then, during the first week of August, conditions again became more favourable for hurricanes and Hurricane Debby was formed. After Debby, the Caribbean expe­rienced Hurricane Ernesto, writes KNMI. Ernesto was a tropical storm as it passed by Saba, St. Maarten and St. Eustatius, but did cause a lot of precipitation, resulting in flooding.

But since Ernesto in mid-August, things have been quiet in the Atlantic, with no more tropical storms. And this despite of predictions of an above-average hurricane season and it being the peak of hurricane season right now. Possible reasons for the unexpectedly quiet hurricane season have been identified by meteorologists.

Firstly, the tropical wave train seems to have shifted. Hurricanes, explains KNMI, especially severe hurricanes, often begin as a tropical wave: a collection of thun­derstorms that originate over the African mainland and move into the Atlantic Ocean near Cape Verde.

But, in recent weeks this train has shifted. The thun­derstorms now suddenly moved much more north­westward, toward the Saha­ra. As a result, heavy rain fell in Morocco, Mauritania and Mali, among others. A num­ber of tropical waves moved into the Atlantic Ocean in the area west of Mauritania and Morocco. But conditions in this area are precisely un­favourable for tropical waves due to colder water and drier air.

Secondly, the troposphere above the Atlantic Ocean is warmer than usual. As a result, the difference in tem­perature between the (sea) surface and high in the atmo­sphere is less than normal. This makes the atmosphere more stable and thus air can­not rise as easily. Possible causes of this warming are the effects of last year’s El Nino, and the long-term ef­fect of climate change, writes KNMI.

Thirdly, there is currently too much vertical wind shear. A hurricane trying to form would prefer as little verti­cal wind shear as possible. This way the storm remains intact and can continue to intensify well. But in recent weeks there has been a lot of wind shear over the Atlantic. In the eastern Atlantic this is probably related to the tropi­cal wave train that has shift­ed. More toward the central and western Atlantic, this vertical wind shear is related to the Madden-Julian Oscil­lation (MJO).

But, warns the KNMI, the very factors that are current­ly causing less hurricane ac­tivity are expected to become more favourable for hurri­canes to form again toward mid-September. For exam­ple, vertical wind shear will become a lot less, making it easier for tropical waves to grow into hurricanes. In addition, the tropical wave train is expected to recover, allowing tropical waves to move back up the Atlantic instead of toward the Sahara.

The hurricane season will last until November 30, KNMI stated in concluding its article.

The Daily Herald.